1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device having, in a semiconductor chip thereof, a voltage generating circuit for generating an operation voltage required for an integrated circuit thereof, a method of investigating a cause of a failure if the failure takes place in the semiconductor integrated circuit device, and a method of verifying the operation of the semiconductor integrated circuit device.
2. Description of the Related Art
An EEPROM, which is one type of non-volatile memory, permits data to be electrically written thereto and data to be electrically erased therefrom. A portion of EEPROM, for example, an EEPROM called NAND EEPROM, uses a tunnel current when data is electrically written or erased. The NAND EEPROM has a plurality of memory cells (hereinafter called "cells") serially connected between bit lines and the ground lines. A portion of the NAND EEPROM comprises, in the chip thereof, a voltage generating circuit for generating writing/erasing voltage, the level of which is higher than the power supply voltage. The NAND EEPROM can be operated by only supplying one type of power supply voltage.
FIGS. 28A to 28C show a cell of the NAND EEPROM. FIG. 28A is a plane view, FIG. 28B is a cross-sectional view taken along line B--B shown in FIG. 28A, and FIG. 28C is a cross-sectional view taken along line C--C shown in FIG. 28A.
FIG. 28A shows two cells, in series, connected to each other. Then, the structure of the cell will now be described while paying attention to one of the two cells.
As shown in FIGS. 28A to 28C, an N-type silicon substrate 1 includes a P.sup.- type well 2 formed therein. The well 2 includes a plurality of N.sup.+ type diffusion layers 3 formed therein. The N.sup.+ type diffusion layers 3 serve as sources and drains of the cell. A portion of the substrate 1 between the N.sup.+ type diffusion layers 3 is used as a channel region 4. On the channel region 4, there is formed a gate oxide film (SiO.sub.2) 5. Note that a thick silicon dioxide film (SiO.sub.2) 6 formed on the well 2 is a field insulating film for separating the cells from one another. The field insulating film 6 is formed by LOCOS (Localized Oxidation of Silicon). A floating gate 7 is formed on the top surface of the gate oxide film 5 to the field insulating film 6. The floating gate 7 is made of electrically conductive polysilicon. The floating gate 7 is a charge storage layer for enabling the threshold of the cell to be variable. Therefore, the floating gate 7 is formed for each cell. On the floating gate 7, there is formed a control gate 9 through an interlayer insulating film (SiO.sub.2). The control gate 9 is made of electrically conductive polysilicon. The control gate 9 serves as a word line.
The method for writing data "0" in the cell will now be described.
The well 2 and the N.sup.+ type diffusion layers 3 respectively are grounded, and then program potential VPP (about 20V) is applied to the control gate 9. As a result, the control gate 9 and the floating gate 7 are capacity-coupled to each other so that the potential of the floating gate 7 is raised. The conduction type of the channel region 4 is reversed from P type to N type. The N-type channel region 4 is connected to the N-type diffusion layer 3. The potential of the channel region 4 is made to be the ground potential. Thus, the potential difference takes place between the channel region 4, which has been made to be the ground potential, and the floating gate 7. Therefore, a tunnel current flows from the floating gate 7 to the channel region 4 (and the N.sup.+ type diffusion layers 3). When the tunnel current has been allowed to flow, electrons are injected into the floating gate 7. The floating gate 7, into which electrons have been injected, is negatively charged. Wherein the floating gate 7 has been negatively charged, the threshold of the cell is raised. When the threshold of the cell has been raised, the cell is turned off when data is read out. The foregoing state is a state in which data "0" has been written. When data is read out, the state in which the cell has been turned on, is a state in which data "1" has been written. In this specification, the method of writing data "1" is omitted from description.
The magnitude of the tunnel current depends upon the level of the potential between the floating gate 7 and the channel region 4. Since change in the magnitude of the tunnel current causes the amount of electrons to be injected into the floating gate 7 to be changed, the amount of charge of the floating gate 7 is changed. That is, even if the same program potential VPP is applied to the control gate 9, change in the intensity of the electric field E results in the threshold of the cell being changed.
The electric field E is represented by the following equation: EQU E={C.sub.CF /(C.sub.CF +C.sub.FS)}.times.(1/t.sub.GAOX).times.V (1)
wherein C.sub.CF is the capacity of a capacitor between the control gate 9 and the floating gate 7, C.sub.FS is the capacity of the capacitor between the control gate 9 and the channel region 4, t.sub.GAOX is the thickness of the gate oxide film 5 and V is the voltage to be applied to the control gate 9.
An assumption is performed that a capacitor having capacity C.sub.CF and a capacitor having capacity C.sub.FS are parallel-plate type, capacitors respectively having areas S.sub.CF and S.sub.FS. Moreover, thickness of the interlayer insulating film 8 is assumed to be t.sub.INTER.
Assuming that the gate oxide film 5 and the interlayer insulating film 8 are made of the same material (SiO.sub.2) having the same dielectric constant, the foregoing Equation (1) can be converted into the following equation: EQU E=(V/t.sub.GAOX).times.[1/{1+(S.sub.FS /S.sub.CF).times.(t.sub.INTER /t.sub.GAOX)}] (2)
As can be understood from Equation (2), the electric field E is in inverse proportion to the thickness t.sub.GAOX and the area S.sub.FS. The area S.sub.FS is determined by gate width W shown in FIG. 28B and gate length L shown in FIG. 28C.
The thickness t.sub.GAOX of the gate oxide film 5 is determined in an oxidizing process for forming the gate oxide film 5. The gate length L is determined in a lithography process for patterning the control gate 9 and the floating gate 7. That is, each of the thickness t.sub.GAOX, the gate width W and the gate length L unintentionally contains dispersion (dispersion occurring during the manufacturing process) from a designed value. Since each of the thickness t.sub.GAOX, the gate width W and the gate length L contains dispersion from the designed value as described above, the electric field E cannot be constant for all chips.
However, since the voltage V is fixed for all chips, the quantity of electrons to be injected into the floating gate 7 is dispersed for each chip.
The dispersion becomes greatest for each manufacturing lot because the same manufacturing conditions cannot be allowed to reappear for all lots even if the manufacturing process is performed on the same manufacturing line.